1. The Field of the Invention
The present invention relates to solid state laser materials and, more particularly, to a novel defect pair in alkali halide crystals that is formed by a molecular impurity in combination with a nearest neighbor color center. The material thus formed is capable of laser emission in the infrared wavelength range under excitation with visible light.
2. Description of the Prior Art
A number of solid state lasers have been developed in the last two decades. These lasers are based exclusively on electronic transitions of impurities in various crystalline or glass materials. Among those are, for example, color center lasers using strongly phonon broadened electronic transitions of F-center-like defects in alkali halide crystals for tunable IR laser operation. Presently the total tuning range accessible with sufficiently powerful primary solid state laser sources extends from the far red portion of the spectrum to the near infrared at about 3.5 .mu.m. Although the wavelength range beyond 3.5 .mu.m can be covered, in principle, with diode lasers and optical parametric oscillators, these light sources have either a very low output power and high spatial beam divergence or are very complicated to operate. In view of the large application potential of near infrared lasers for numerous uses (including, for example, molecular spectroscopy of a wide variety of organic and inorganic molecules, pollution detection, and photochemistry), a strong need exists to develop superior laser systems in the infrared range.
A new approach to the development of IR-lasers became possible after the discovery of vibrational emission from dilute molecules immersed in alkali halide host crystals, reported by Y. Yang and F. Luty in Phys. Rev. Lett. 51, 419 (1983) and W. Gellermann, K. Koch, Y. Yang, and F. Luty, Bull. Am. Phys. Soc., 28, 452 (1983). They were first to observe a strong infrared emission around 4.8 .mu.m originating from the vibrationally excited molecular impurities of CN.sup.- molecules in the host crystal KCl. Furthermore, they found that after association of F-centers to the CN.sup.- defects the emission could be efficiently and conveniently pumped by optical F-center excitation in the visible wavelength range through electronic-vibrational coupling. Besides the scientific interest in these first discovered vibrational emission effects in ionic solids, "the possibilities for laser applications, which are under study" were pointed out and discussed in the first conference report on these systems (Bull. Am. Phys. Soc. 28, 452 (1983).
A first realization of this application potential of these new solid state infrared emitters was afterwards demonstrated by R. W. Tkach, T. R. Gosnell and A. J. Sievers at Cornell University, who reported in Optics Letters 10, 122 (1984) laser oscillation at 2054 cm.sup.-1 from CN.sup.- molecules in uncolored KBr host crystals. Pumped by 300 .mu.J pulses from a frequency-doubled C0.sub.2 laser with 100 ns pulse width, population inversion was produced between the second and first vibrational energy level of the CN.sup.- molecule. Using gold coatings directly on the crystal to provide optical feedback they observed CN.sup.- laser oscillation up to a maximum temperature of 4K.
In a later paper by the Cornell group which appeared in Optics Letters 10, 125 (1985), the same host material was operated as a continuous wave laser, pumped with a color center laser to the first overtone molecular energy level. Population inversion was produced in this way directly between the second and first vibrational energy level. Laser oscillation on this v=2.fwdarw.1 transition could be obtained up to a maximum temperature of 4K.
The discovered novel vibrational emission of dilute molecular systems in alkali halide crystals has already been shown to open up new possibilities for IR laser developments. However, the laser systems realized so far with CN.sup.- doped KBr crystals are still impractical for application purposes. The used pumping scheme, i.e., direct optical excitation of the first overtone level of the molecule, is rather inefficient due to the lower absorption strength (by two orders of magnitude) of this transition compared to the fundamental absorption. Furthermore, the achieved output power is only in the microwatt range and laser operation is limited to impractical low temperatures below 4K, requiring complicated and expensive crystal cooling techniques.